1. Field of the Invention
The present invention relates to a piston and, more particularly, to a low mass piston adapted for use in pressurizing a material to be dispensed from an aerosol container.
2. Description of Related Art
Aerosol containers are used to dispense many materials, all of which, by definition, are held under pressure in the container. In some cases, a piston is disposed within the container, and the material to be dispensed is on one side of the piston and a pressurized fluid, typically air, is on the other side of the piston. As the material is dispensed, the piston maintains pressure on the remaining material by translating longitudinally within the container in contact with the inside wall of the container.
For proper operation, the piston must form and maintain an effective seal with the inside wall of the aerosol container. If the piston fails to seal, the material to be dispensed may leak to the pressurized fluid side of the piston. This leakage reduces the amount of material which can be dispensed. Moreover, for certain types of material and pressurized fluid, the leaked material may spoil. Additionally, when the piston-sidewall seal fails, the pressurized fluid may leak to the material side of the piston. This fault, known as blow by can also create problems.
Discontinuities in the inside wall of an aerosol container make it difficult to maintain an effective seal between the piston and the side wall. Discontinuities can be either consistent (e.g., a seam) or random (e.g., a dent). Such discontinuities can cause the seal to fail or the piston to bind or both. The likelihood of either seal failure or piston binding is dependent on both the longitudinal and radial rigidity of the piston. That is, a piston having a high radial rigidity is likely to leak or bind when it encounters a discontinuity. A piston having a high longitudinal rigidity is likely to bind when it encounters a discontinuity.
Many different piston designs have been proposed in attempts to provide an effective seal for an aerosol container, but the simplest, least expensive, and therefore most desirable design is a piston having a flexible skirt. Accordingly, a common piston configuration is a one-piece injection molded plastic piston having a face portion and a flexible skirt for sealingly engaging the inside wall of the aerosol container. The longitudinal and radial rigidity of the piston are generally determined by the length and the thickness of the plastic skirt.
Injection molding, however, inherently limits how thin the skirt can be made. If the skirt is made too thin, molten plastic will not consistently and evenly fill the mold. Pistons having longitudinal ridges for channeling the molten plastic into the thin skirt walls are known in the art (see, for example, U.S. Pat. No. 3,915,352), but such ridges materially increase the longitudinal rigidity of the skirt. Other patents also show pistons having ridges in the piston skirt (see, for example, U.S. Pat. No. 3,099,370 and U.S. Pat. No. 3,132,570), but all such ridges will materially increase the rigidity of the skirt.